1. Field of Invention
The present invention relates to a LED (light emission diode) driver, and more particularly to a control circuit for controlling the LED.
2. Description of Related Art
The LED driver is utilized to control the brightness of LED in accordance with its characteristics. The LED driver is utilized to control the current that flows through the LED. A higher current increases the intensity of the brightness, but decreases the life of the LED. FIG. 1 shows a traditional circuit of the LED driver. The voltage source 10 is adjusted to provide a current ILED to the LEDs 20˜25 through a resistor 15. The current ILED can be shown as equation (1):
                              I          LED                =                              V            -                          V                              F                ⁢                                                                  ⁢                20                                      -                          V                              F                ⁢                                                                  ⁢                21                                      -            …            -                          V                              F                ⁢                                                                  ⁢                25                                                          R            15                                              (        1        )            
wherein the VF20˜VF25 are the forward voltage of the LEDs 20˜25 respectively.
The drawback of the LED driver shown in FIG. 1 is the variation of the current ILED. The current ILED is changed in response to the change of the forward voltages of VF20˜VF25. The forward voltages of VF20˜VF25 are not a constant due to the variation of the production and operating temperature. The second drawback of the LED driver shown in FIG. 1 is the power loss occurred on the resistor 15.
FIG. 2 shows another traditional approach of the LED driver. A current source 35 is connected in series with the LEDs 20˜25 to provide a constant current to the LEDs 20˜25. However, the disadvantage of this circuit is the power loss of the current source 35, particularly, as the voltage source 30 is high and the LED voltage drops of VF20˜VF25 are low. Besides, a chromaticity and a luminosity of the LED are affected by the change of the LED's operating temperature. In order to keep the chromaticity and/or the luminosity of the LED as a constant, the current of the LED should be adjusted in response to the change of the temperature. The major objective of the present invention is to provide a LED driver to achieve a higher efficiency. The second objective of the present invention is to develop a LED driver capable of compensating the influence of the temperature.